diff --git a/src/app/file/png_format.cpp b/src/app/file/png_format.cpp
index 797a597ee..f69200fdd 100644
--- a/src/app/file/png_format.cpp
+++ b/src/app/file/png_format.cpp
@@ -446,6 +446,7 @@ bool PngFormat::onSave(FileOp* fop)
       mask_entry = fop->document()->sprite()->transparentColor();
     }
 
+    bool all_opaque = true;
     int num_trans = pal_size;
     png_bytep trans = (png_bytep)png_malloc(png_ptr, num_trans);
 
@@ -453,9 +454,13 @@ bool PngFormat::onSave(FileOp* fop)
       int alpha = 255;
       fop->sequenceGetAlpha(c, &alpha);
       trans[c] = (c == mask_entry ? 0: alpha);
+      if (alpha < 255)
+        all_opaque = false;
     }
 
-    png_set_tRNS(png_ptr, info_ptr, trans, num_trans, NULL);
+    if (!all_opaque || mask_entry >= 0)
+      png_set_tRNS(png_ptr, info_ptr, trans, num_trans, nullptr);
+
     png_free(png_ptr, trans);
   }